When use is made of raw-material gases for the purpose of producing chemical products, the purity requirements placed on the respective raw-material gas are generally different from product to product.
For instance, chlorine is employed as raw material in the production of isocyanates or polycarbonate. Chlorine is usually contaminated with bromine, which in the case of the production of certain isocyanates has to be almost completely removed from the chlorine. The effort required for this is considerable, since bromine and chlorine can only be separated from one another with difficulty.
Processes for producing polycarbonate, on the other hand, are generally more tolerant with respect to the bromine content in the chlorine. In a chemical site chlorine is usually supplied from a common source of raw material, for example a raw-material storage tank, which provides the entire site via a network of pipes. Therefore the effort for purifying the chlorine has to be in line with the product having the highest purity requirement placed on the chlorine.
With reference to the process that is more bromine-tolerant, unnecessarily high expenditure is accordingly incurred, for this process is also supplied with clean chlorine. Moreover, some of the raw material, chlorine, is discarded together with the separated bromine, resulting in losses of raw material.
In the course of the purification of the raw material, chlorine, it is to be noted in addition that chlorine originating from electrolytic processes contains, more often than not, nitrogen trichloride (NCl3) as high-boiling contaminant. Since NCl3 is an explosive substance which at relatively high concentrations has a tendency towards spontaneous decomposition, the maximal concentration thereof in the chlorine has to be safely limited. Explosive substances in the sense of the present invention are NCl3 and also all substances that have the GHS coding GHS01 (GHS, Globally Harmonized System of Classification and Labelling of Chemicals, adopted by the United Nations) and/or hazardous-materials Class 1 and/or 5.2 according to the European Agreement concerning the International Carriage of Dangerous Goods by Road (ADR). In particular, explosive substances in the sense of the invention are those of the aforementioned substances, which in comparison with the raw material to be purified are high-boiling, that is to say, have a lower vapour pressure under otherwise identical conditions.
In the course of a fractionation by means of distillation for the purpose of purifying chlorine, NCl3 is enriched, in accordance with its low vapour pressure as a high-boiling component, at the bottom of the column together with bromine. At the head of the column the bromine-lean, low-boiling chlorine fraction is then drawn off.
Now if the demand for the bottom fraction is low, the fractionation is designed in such a way that only a small bottom stream and a correspondingly large head stream arise. This means, however, at the same time that the proportion of NCl3 in the bottom fraction rises and may fall into a range that constitutes a safety risk.
If it is now desired to provide the bottom fraction likewise in gaseous form, said fraction has to be evaporated. In the course of this evaporation the lower-boiling components will at first pass over into the gas phase, and the high-boiling components will firstly remain in the liquid phase. In the process, the NCl3 will be enriched still further in the remaining liquid phase, so that the safety risk rises further.
From the state of the art a number of processes for purifying chlorine are known, in the course of which bromine is removed.
For instance, EP 1 947 054 A1 discloses the separation of bromine from chlorine, wherein a stream of chlorine gas is conducted upwards from below through a wash column in which liquid chlorine charged at the top trickles towards said stream. The stream supplied at the bottom and the stream charged at the top must be in a ratio from 1/1 to 1/0.3. This process can be applied for the generation of a purified, bromine-lean stream of chlorine gas, and of a liquid bromine-rich stream of chlorine, the mass-flow ratio of these two streams being very restricted. Said process is therefore not suitable for the flexibility requirements of a modern chemical site. In addition, no description is given of how a gaseous stream can be generated from the stream of liquid. This, in itself, trivial step is very elaborate, by reason of stringent safety requirements, and has to be designed appropriately.
WO 2011/058069 A2 discloses a process for distillatory purification of a stream of chlorine. In this process, gaseous chlorine with less than 20 ppm bromine as a purified stream and also a liquid, bromine-containing stream of chlorine are generated. The process has been designed in such a way that the effort for pipelines and external condensers has been reduced. Moreover, additional safety is gained, since at the same time the holdup for liquid chlorine, bromine and NCl3 is also reduced.
WO 2011/058069 A2, however, discloses neither a limitation in the ratio of the streams of chlorine supplied at the bottom and charged at the top, nor whether or how the liquid stream of chlorine can optionally be converted into a gaseous stream of chlorine.
WO 2004/018355 A1 discloses a process and a device in order to separate bromine from chlorine gas. The gas is routed into a distillation column from which a purified stream of chlorine gas is withdrawn as head product. The purified stream of gas is compressed and subsequently conducted into a condenser and partially liquefied. The condenser has been thermally integrated with the bottom evaporator of the distillation column. The liquid bromine-rich bottom stream out of the column is either purged or routed into a subsequent evaporator and evaporated.
The gaseous stream from this evaporator is purified further in a second distillation column. From the latter a highly bromine-containing stream of liquid is ejected from the bottom evaporator. At the head a stream of gas issues which is conducted back into the first column.
However, WO 2004/018355 A1 does not deal with the special safety problems in connection with the evaporation of an NCl3-containing stream of chlorine.